Wild Growth was changed a little in its healing priority this expansion. The spell will decide who to heal at cast completion, but it does not always prioritize the lowest health targets, and will randomly heal anyone within range of cast who is hurt (not 100% hp).

In terms of Gruul, this can be handled by optimizing your target on each stack target. Try and position yourself so that you are out of range of the undesired group and WG yourself, or when your side is not stacking, you can move away from your stack point and heal someoen farther back on the other side. This is kind of dependent on how your group positions stack points for Gruul though.

For my opinion on Oregorger, you should aim to start casting Wild Growth a little before Acid Torrent will hit your Ranged stack so that the Growth goes out as Acid Torrent is hitting. The reasoning is that Wild Growth will do most of its healing at the beginning, so you want the ticks to happen as the players start to take the damage. Wild Growth too early and the ranged stack will only get the smaller heals.

Here's a post from about a year ago regarding the ideal changes to spells like Wild Growth: http://manaflask.com/en/articles/dev-watercooler-building-healthy-gameplay

Another of our goals for healing in this expansion is to strike a better balance between single-target and multi-target healing spells. We've taken a close look at the mana efficiency of our multi-target heals, and in many cases, we're reducing their efficiency, usually by reducing the amount they heal. Sometimes, but more rarely, raising their mana cost was a better decision. We want players to use multi-target heals, but they should only be better than their single-target equivalents when they heal more than two players without any overhealing. This way, players will face an interesting choice between whether to use a single-target heal or a multi-target heal based on the situation.